Carotenoid- and melanin-based coloration have been indicated as biomarkers of individual quality and as traits associated with sexual selection in many bird species. Though not mutually exclusive, coloration of birds may also contribute to camouflage, signaling of species identity or social membership, temperature regulation, or in anti-predator defense. Within these different aspects of avian coloration, there is often significant variation among members of the same species; thus, the coloration of a bird may be an honest indicator of physiological condition. We assessed relationships between melanin-based plumage coloration and physiological profiles in the white-breasted nuthatch (Sitta carolinensis). Throughout most of the species’ range, male white-breasted nuthatches have dark black caps, while females and recently-fledged hatch year birds have lighter gray caps. We captured white-breasted nuthatches from June to September 2020, quantified cap color, cap size relative to head size, and analyzed blood samples for leukocyte counts, parasite loads, testosterone levels, and antioxidant capacity. While the diversity in cap color and size differs between sexes, we found considerable variation in cap color and size within groups of male and female nuthatches. Our findings may reveal underlying mechanisms in expression of the melanin-based cap coloration in this species.